In known apparatus for forming a musical-tone signal for electronic musical instruments, a musical-tone-signal waveform is formed and is passed through an envelope circuit for forming a musical-tone signal having a particular envelope. This envelope circuit generally requires a large-capacity condenser and therefore the circuit must be of large size. Furthermore, the available envelope is limited to a particular range because of the necessity of limiting of the capacity, so that it is generally impossible to form many of the envelopes characterizing various kinds of natural musical instruments or to form other envelopes of original design.